1. Field of the Invention
The present invention relates to a method and device for processing an optical signal.
2. Description of the Related Art
By inputting an optical pulse having a high power level into a nonlinear optical medium such as an optical fiber, spectrally broadened light called supercontinuum (SC) light can be generated. The application of SC light to a multiwavelength light source has been actively examined in the prior art, and any other applications of SC light to multiwavelength optical clock regeneration, wavelength conversion, distribution, etc. are also possible.
Optical clock pulses having repetition time to or a data-modulated optical pulse train are/is input into an optical fiber to generate SC light. In this case, SC light having a band spectrum configured by continuous-wave (CW) light having a frequency spacing f=1/t0 is generated. By extracting each CW light in the SC light by using a narrow-band optical bandpass filter, a multiwavelength CW light source can be configured. Further, by using an optical bandpass filter capable of passing a plurality of CW lights, multiwavelength optical clock pulses having repetition time to can be generated. In the case of inputting a data-modulated optical pulse train into an optical fiber to generate SC light, wavelength-converted light of data signal light, for example, can be generated by using an optical bandpass filter. The pulse width of the optical clock or wavelength-converted light generated can be controlled according to the passband of the optical bandpass filter used.
An optical fiber having a length required for generation of necessary third-order nonlinear effects is used for the generation of SC light in the prior art. To broaden the spectrum as greatly as possible, a fiber having a small anomalous dispersion or a dispersion flattened fiber, for example, is used. To broaden the spectrum and ensure a sufficient signal-to-noise ratio (S/N ratio), a fiber having a small normal dispersion, for example, is used.
SC light is generated by using self phase modulation (SPM) in an optical fiber as a nonlinear optical medium. Accordingly, a short pulse having a steep slope is suitable for the optical pulse as the SC light. However, the effect of dispersion becomes more remarkable with a decrease in pulse width, and it is therefore difficult to maintain an optimum pulse shape over the length of the optical fiber for generating the SC light. As a result, there is a limit to the fiber length allowing the generation of the SC light, causing a limit to a spectral broadening rate.
Known as a method of maintaining a pulse shape in an optical fiber is a method of using a dispersion decreasing fiber (DDF) such that dispersion is decreased in an anomalous dispersion region in accordance with attenuation of power. However, the design and manufacture of this fiber are difficult, and the optical power must be maintained at a specific value, so that this method is difficult to be practical.
A more practical generating method for SC light is to use an optical fiber having normal dispersion. In this case, the sign of chirp induced by SPM is reversed at the opposite edges of an optical pulse, so that the pulse is rectangularly changed by dispersion and the spectrum is also rectangularly broadened. Accordingly, the S/N ratio can be maintained good. However, the spectral broadening rate is limited as compared with the case of using an anomalous dispersion fiber. Consequently, it is desirable to achieve an SC light generating method which can maintain the S/N ratio by using a normal dispersion fiber and can enlarge the spectral broadening rate.